This invention relates to a rotary gas compressor which comprises an impeller housing, an impeller rotatably mounted in the housing, a housing cover, a control disc arranged between the housing and the cover, a suction intake, a pressure outlet, a suction slot arranged in the control disc to provide communication between the suction intake and the impeller, a pressure slot arranged in the control disc to provide communication between the impeller and the pressure outlet, and a pressure relief arrangement for supplying ballast gas to the compressor when required.
A compressor of this type is disclosed in German Pat. No. 284 674. In the pressure chamber of this compressor is a tube attached to an aperture provded in the control disc, through which with specific pressure ratios ballast gas can be introduced into the cells of the impeller so that at the pressure slot a pressure corresponding as far as possible to the atmospheric pressure is achieved. Since with a still low vaccum at the intake stack, the gas to be supplied reaches a pressure exceeding the atmospheric pressure substantially before reaching the pressure slot, part of the gas to be supplied and possibly even liquid would escape via the tube. Therefore, installed in the tube is an automatically operating non-return valve by means of which such an escape is prevented.